In order to better characterize the invention and the problem which it solves, five categories of existing polyamide materials will be mentioned. The term “polyamide materials” is understood to mean compositions based on polyamides, copolyamides and polyamide alloys or based on polyamides.
(1) Impact-Modified Polyamide Materials (High-Impact PA)
These are alloys of polyamide with a minor amount of elastomer, typically in the vicinity of 20% by weight. The polyamide is typically a semicrystalline polyamide. These alloys have the advantage of a very good impact strength, much improved with respect to polyamide alone, typically three times better or more. They also have good chemical resistance and satisfactory resistance to distortion under heat (60° C.). They have the disadvantage of being opaque, which can be a problem for decorative components. A well-known example of high-impact polyamide is “Zytel ST801” from DuPont.
(2) Transparent Amorphous Polyamide Materials (TR amPA)
These are materials which are transparent, which are amorphous or not very semicrystalline (enthalpy of fusion during the DSC second heating of less than 30 J/g), which are rigid (flexural modulus ISO>1300 MPa) and which do not distort under heat, at 60° C., as they have a glass transition temperature Tg of greater than 75° C. However, they are rather unresistant to impacts, exhibiting a much lower notched Charpy ISO impact in comparison with impact-modified polyamides, and their chemical resistance is not excellent, in particular due to their amorphous nature. There also exists (but these are materials less frequently encountered) transparent semicrystalline (or microcrystalline) polyamides, typically with enthalpies of fusion during the DSC second heating between 2 and 30 J/g, these materials also being fairly rigid and having a flexural modulus ISO>1000 MPa.
(3) Polyether-Block-Amide and Copolymers Comprising Ether and Amide Units (PEBA)
These are copolyamides based on ether units and on amide units, polyetheramides and in particular polyether-block-amides (PEBAs). These are very flexible highly impact-resistant materials but with a fairly low transparency (45 to 65% of light transmission at 560 nm for a thickness of 2 mm), just like their polyamide homologues without ether units.
(4) Semicrystalline Polyamides (PA)
These are typically linear aliphatic polyamides. Their crystallinity is reflected by the presence of spherolites, the size of which is sufficiently great for the material not to be highly transparent (light transmission of less than 75%).
(5) Transparent Semicrystalline Polyamides (TR scPA)
These are more specifically microcrystalline polyamides where the size of the spherolites is sufficiently small to retain the transparency; (see European Patents EP 550 308 and EP 725 101) (transparency>75%).
The various properties of the five categories of polyamides which have just been indicated have been summarized in Table 1A below:
TABLE 1AImpact/Tempera-ChemicalPolyamideTransparencybreakingFlexibilitytureresistanceElasticProcessingcategory(a)strength (b)c)stability (d)(e)fatigue (f)(g)(1) High-impact−−−+++++++++++++PA(2) TR amPA(1)+++−− to −−−++ to +++− to −−−−− to +−(3) PEBA(2)−− to ++ to ++++ to ++++ to +++ to +++++++ to +++(4) PA−− to −+++++++++++(5) TR scPA++ to ++++− to +− to ++ to +++− to +−Grades from −−− = very bad to +++ = very good